The aim of the study is to prove that this test is as equally effective as the allograft biopsy to detect acute rejection in patients that undergo a biopsy for unexplained renal dysfunction. The perspective of this approach is that the test could be used either in place of the biopsy or as decision guidance whether a biopsy is necessary to confirm the presence of rejection. Another perspective is that the MS test (respectively, a simplified test system derived from this method) could be used in the regular post-transplant surveillance for acute rejection, in place of the relatively insensitive procedure with periodic monitoring of the graft function by creatinine determinations.